To say that the phenomenon of
synesthesia is interesting is to understate the point and Richard E. Cytowics
second edition of Synesthesia: A Union of
the Senses does the topic due justice.This second edition involves more phenomenological accounts than
did the first with additions and revisions made on the basis of new research
into synesthesia and into human brain functioning and development
generally.The book begins with an
introduction to certain historical issues, but quickly moves to a thorough and
interesting discussion of synesthesia case studies in the second chapter.Colored
hearing and colored letters and numbers are the most common types of
synesthesia (p. 7), but there are a wide variety of synesthetic
experiences.For subject TP, not only
do letters and numbers have color, but music does as well; in other words, the
auditory perception of music evokes color experiences in TP (pp. 25-26).MT experiences letters and numbers not only
as being colored but also as having gender and personality; the letter M, for
example, is perceived to be blue-violet, male, and powerful (p. 297).CSc, on the other hand, experiences
involuntary tastes and smells while playing the piano or oboe.Indeed, the tastes are sometimes so intense
that they interfere with her musical concentration (p. 27).DS sees anger not as red  as is the popular
metaphor  but as purple.If Im
really upset at my kids and Im yelling at them, there will be a purple
background behind them (p. 25).MW
perceives weight, shape, texture, and temperature when he tastes something with
an intense flavor.He describes the
taste of spearmint, for example, as cool glass columns (p. 25).Indeed, he utilizes these tangible sensory
experiences in his cooking; so, for example, chicken must be cooked until it
has the appropriate number of points (p. 6).

The reader might be tempted to explain these sorts of reports in terms
of a sophisticated use of metaphor.After all, many of us would say that anger makes us see red or that
certain sounds have certain colors.However, as Cytowic makes clear, this is not a use of metaphor.TP really does have color experiences that
accompany his perception of letters, numbers, and music, and MW, tangible
sensory experiences when tasting food.The third chapter takes up the issue of the diagnostic criteria
for synesthesia.Most importantly, synesthetic experiences
are 1) involuntary sensory experiences in one sense modality that
accompany the stimulation of another, and 2) are consistent over
time.So for the synesthete, sensory
stimulation in one sense modality leads to an automatic experience in another
that is consistent and reproducible over her lifetime.

The fourth chapter looks at related neuropsychological phenomena,
including the effects of certain drugs, release and sensory deprivation
hallucinations, and the subjective experiences that often accompany temporal
lobe epilepsy.The fifth chapter  new
to the second edition  takes up a discussion of spatial extension as it
relates to synesthetic perception.According to Cytowic, it is an essential element of synesthetic
experiences that they are experienced as extended in space.The synesthetic percept is taken to exist
out there (p. 68) rather than in the imagination.So, for example, if visual, it will be experienced as if on a
screen in front of the subjects face.The sixth chapter considers the neural substrate of synesthesia.Cytowic contends that the limbic system
plays an important role in the binding the activities of the areas of the brain
involved in synesthetic experience.Cytowic here and elsewhere in the book emphasizes the extent to which
synesthetic percepts are emotionally valenced.The seventh chapter  new to the second edition  looks at developmental
issues, including Daphne Maurers interesting, if controversial, claim that
neonates are inherently synesthetic.The penultimate chapter includes an interesting discussion of the
personalities of synesthetes.The areas
of memory and mathematical reasoning as well as artistic and aesthetic
expression, among others, are considered.The book ends with a discussion of vision and color perception as they
relate to synesthesia; specifically, Cytowic considers the theory of microgenesis
 a theory that, among other things, challenges the conventional view of object
representation as a building up of objects out of sense data (p. 342)  and how
it may relate to hallucination, visual illusion, and, of course, synesthesia.

The critic will likely note the sometimes speculative nature of the
discussion as well as the potentially controversial interpretation of empirical
data.Nevertheless, as a well written
and comprehensive discussion of case studies, and past and contemporary research
into the neurophysiological mechanisms that underpin synesthesia and related
phenomena, Synesthesia: A Union of the
Senses is useful both for the novice and advanced reader interested in
consciousness, perception, and emotion.

Liam Dempsey is a doctoral
candidate (ABD) in the department of philosophy at the University of Western
Ontario.His interests include
philosophy of mind and the metaphysics
of qualia, philosophy of psychology, and cognitive science.